1. Field of the Invention
The present invention relates to a diversity reception apparatus which, in digital radio communications and the like, takes out demodulated data in two or more channels from one or a plurality of modulated signals and selects and combines the data to reduce transmission errors.
2. Description of the Background Art
In radio communications, especially in mobile communication, the diversity reception is widely used to reduce transmission errors caused by fading and interference.
In a typical method of the diversity reception, a signal at the highest reception signal strength is selected from among a plurality of channels of signals received at a plurality of antennas. However, in the presence of interference or multipath, error may occur even if the reception signal strength is high. Then a channel of signal of good quality cannot be correctly selected only with the reception signal strength. Conventionally, in order to solve this problem, Japanese Patent Laid-Open No. 1-265739 and Japanese Patent Laid-Open No. 4-8031 propose diversity reception apparatus which can detect errors in received data to select a channel containing the smallest number of errors. Such conventional diversity reception apparatus will now be described referring to the drawing.
FIG. 17 is a block diagram showing the structure of a conventional diversity reception apparatus. As shown in FIG. 17, this diversity reception apparatus has antennas 101a and 101b, receivers 102a and 102b, error detectors 103a and 103b, an error comparator 104, and a data selector 105. The receivers 102a and 102b demodulate signals received at the antennas 101a and 101b to output demodulated data d100a and d100b, respectively. The error detectors 103a and 103b respectively detect errors in the demodulated data d100a and d100b and count the numbers of errors. The error comparator 104 receives and compares the numbers of errors outputted from the error detectors 103a and 103b and outputs a selection signal s100 for selecting one of the demodulated data d100a and d100b containing less errors. The data selector 105 selects the one with less errors from the demodulated data d100a and d100b on the basis of the selection signal s100 from the error comparator 104 and outputs it as a selected signal d101. With this operation, the diversity reception apparatus shown in FIG. 17 can select demodulated data with less errors from the signals received at the two channels of antennas.
However, the diversity reception apparatus with the above-described structure requires use of a highly redundant error detecting code or a long error detecting code to correctly detect errors. The use of a long error detecting code reduces the selecting rate and then it is impossible to follow a rapid change on the transmission path caused by fast fading, for example. However, the use of a highly redundant error detecting code reduces the data transmission efficiency. Further, if error correcting coding is used to improve the reliability, the redundancy further increases. Moreover, if the demodulated data in all channels include errors to the same extent, it is impossible to determine which channel of demodulated data has the best quality.